1. Field of the Invention
The present invention generally relates to the field of optical fibers, in particular to the manufacture of fiber optic buffer tubes having a substantially constant excess fiber length (xe2x80x9cEFLxe2x80x9d) ratio throughout the length of the buffer tube.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable (including wire cable) networks. The use of optical fibers in today""s technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Most applications of optical fibers require the individual fibers to be placed into groupings, such as in fiber optic cables.
There are many ways to manufacture and configure fiber optic cables. One of the most common methods of manufacturing a fiber optic cable is by placing a number of fiber optic buffer tubes in a single cable jacket, where each of the buffer tubes is a separate tube having a number of individual optical fibers or optical fiber ribbons. The buffer tubes themselves are hollow tubes generally made from thermoplastic materials.
When a cable construction uses a number of buffer tubes, each containing a number of fibers, the quality of the finished cable greatly depends on the quality of the components it uses, in particular, the buffer tubes. The quality of the individual buffer tubes can be affected by a large number of factors, and the manufacture of the buffer tubes, with the fibers, is one of the most critical of these factors. A common method of manufacturing the buffer tubes is to draw the tubes with the fibers placed inside of the tubes. The buffer tube is then wrapped around a spool and left to cool at room temperature. During this process the tubes are reeled (in the drawing process) onto a hard or rigid spool (made of any sturdy material for example, wood or steel), and are drawn at a constant draw or take-up tension (on the tube itself) and with a constant angular velocity of the spool.
However, when reeling of the buffer tubes occurs with constant tensile draw on the tubes and constant angular speed of the spool the result is non-uniform distribution of residual stresses along the length of the buffer tube as it sits on the spool. In some cases, the non-uniform distribution of EFL remains after taking the tube off the spool and subsequently negatively affecting attenuation in the finished cable. The main components, or origins, of the non-uniform, along the tube length, residual deformation stem from (1) stresses along the reeled buffer tube axis (i.e. circumferential stresses) which is a function of the distance from the reel surface, i.e. current radius and (2) in a transverse direction (i.e. radial stresses) typically varying from zero on the roll surface to a maximum amount on the reel surface. It is noted that this problem not only exists in the fiber optic industry, but also in any other industry where the extended rolling of a product is required. For example, the same problems exist in the manufacture of paper, electrical cable, aluminum sheet, etc. Resulting from these non-uniform stresses on the spool is the permanent or residual deformation of the rolled material (i.e. buffer tubes) and the creation of residual strains in the tubes and the fibers within the tubes. The creation of residual strain in the fibers is a very serious problem in the manufacture of fiber optic cables and buffer tubes that causes variation of EFL along the length of the tubes and subsequently attenuation problems.
Excess Fiber Length (xe2x80x9cEFLxe2x80x9d) is an important parameter affecting the quality and performance of a fiber optic cable. EFL is generally defined as a relative difference between the actual individual fiber length (defined as xe2x80x9cLFxe2x80x9d) and the length of the buffer tube from which the fiber came (defined as xe2x80x9cLBxe2x80x9d), where the % EFL=[(LFxe2x88x92LB)/LB]xc3x97100. EFL is important in the proper operation of a fiber optic cable. In general, it is desirable to have a small positive EFL. This means that the length of the fibers is larger than the length of the buffer tube in which the fibers are disposed. This added length allows delayed stretching of the buffer tube under a tensile load during installation or its use without adding any tensile loads on the fibers meaning that, to a certain level of tensile load, the load will be carried by the strength rods or tapes, and not involve the fibers. However, it is important that the EFL should not be too large and have a relatively even distribution throughout the length of the buffer tube. When the EFL distribution throughout the length of a tube is non-uniform it can adversely affect the operation and efficiency of the cable as a whole.
Through testing, it has been discovered that measurements of EFL in buffer tubes wrapped according to prior art methods results in buffer tubes which show an EFL distribution having a skewed parabolic shape. This is depicted in FIG. 1, which shows a graphical representation of a typical EFL distribution in a buffer tube wound under the prior art methodology. As shown, the EFL curve 1 is of a typical buffer tube length after manufacture. The left side of the graph indicates the EFL at the early stages of the tube manufacture (i.e. the beginning portion of the tube on the reel). The rapid or steep change in the left part of the EFL curve occurs in the tube length during the initial wraps of the tube on the reel. The remaining portion of the curve shows that the EFL peaks at some point near the center of the tube length and then tapers off near the end of the tube winding.
This is a significant problem with long buffer tubes (approximately 10 km in length) and having a relatively small core radius for the take-up reel (around 100 mm). When the parabolic variation becomes too large the fiber attenuation near the middle of the length of the cable can be significant, thus making the cable useless.
An additional problem of the prior art methods of manufacturing buffer tubes is the limiting effect on the line speeds of the manufacturing process due to the uneven EFL distributions. As line speeds increase the EFL distribution problems become more significant. Therefore, to avoid these problems, manufacturing speeds are limited so as to prevent significant EFL problems.
The present invention is directed to eliminating or greatly reducing the impact of the above problems by the use of prior art methods of manufacturing buffer tubes with optical fibers.
The present invention uses the variation of a number of different parameters or physical characteristics of the manufacturing process or equipment, either individually or in combination, to provide a substantially uniform EFL distribution along the entire length of a manufactured buffer tube. In a first embodiment of the present invention, a pad having a compliant stiffness is placed on the core of the take-up reel prior to the winding of the manufactured buffer tube, and the take-up tension of the tube as it is being drawn is monotonically decayed according to a set function so as to ensure an even EFL distribution along the entire length of the buffer tube. Although it is contemplated that the present invention can provide an even EFL distribution without the use of a stiffness-compliant pad, it is to be used in the preferred embodiment to provide stress relief in the initial layers of the buffer tube, located closest to the core.
In a second embodiment of the present invention, a substantially even EFL distribution is accomplished by using a combination of the stiffness compliant pad on the core of the reel with varying the angular speed of the take-up spool during the spooling of the buffer tube. Similar to the first embodiment, the variation in the speed of the take-up spool combined with the stiffness-compliant pad is used to provide a substantially even stress and strain distribution throughout the length of the tube, thus resulting in a substantially even EFL distribution. As with the first embodiment, it is contemplated that only the variation in the angular speed of the take-up spool can be used to provide an even EFL distribution, but in the preferred embodiment the combination is to be used.
In a third embodiment of the present invention, stiffness-compliant pads are placed between layers of tube windings at intervals throughout the reeling of the buffer tube, as well as on the spool core. The use of these pads at intervals allow the excess stress and strain in the tubes and fibers to be absorbed in the pads. It is preferred, in this embodiment, that the use of the stiffness-compliant pads be combined with either varying the take-up tension or the angular speed of the spool, as discussed in the previous two embodiments. Further, in this embodiment, pads can be placed between each winding or at regular intervals in the tube winding. Additionally, this embodiment can be used with a stiffness-compliant pad on the reel core, as described above. It is preferred in this embodiment, that the tubes be re-reeled after the initial reeling step and the tube is allowed to cool to room temperature to aid in achieving a more uniform EFL distribution. The re-reeling step can be used with any of the above embodiments.
In the fourth embodiment, the layers are separated with rigid, preferably metal or composite cylindrical panels separating the layers and thus xe2x80x9cbreakingxe2x80x9d up the stress compounding from upper layers. The panels can have slots to allow the tube to continue onto the next level.
It is to be noted that it is further contemplated that although the above embodiments can be used individually to obtain a substantially even EFL distribution, it is contemplated that any combination of the embodiments, or components thereof, can be used without altering the scope or spirit of the present invention. For instance, monotonically decaying the take-up tension may be combined with the varying of the angular spool speed and the use of stiffness-compliant pads or stiff cylindrical separators in the windings of the spool to achieve an even EFL distribution.